The carbon burner operated in a rarefied hydrocarbon vapor. This lamp is in the collection of the Smithsonian Institution.] Edison’s first experiments were to confirm the failures of other experimenters. Convinced of the seeming impossibility of carbon, he turned his attention to platinum as a light giving element. Realizing the importance of operating platinum close to its melting temperature, he designed a lamp which had a thermostatic arrangement so that the burner would be automatically short circuited the moment its temperature became dangerously close to melting. The burner consisted of a double helix of platinum wire within which was a rod. When the temperature of the platinum became too high, the rod in expanding would short circuit the platinum. The platinum cooled at once, the rod contracted opening the short circuit and allowing current to flow through the burner again. His first incandescent lamp patent covered this lamp. His next patent covered a similar lamp with an improved thermostat consisting of an expanding diaphragm. Both of these lamps were designed for use on series circuits. [Illustration: EDISON’S FIRST EXPERIMENTAL LAMP, 1878. The burner was a coil of platinum wire which was protected from operating at too high a temperature by a thermostat.] The only system of distributing electricity, known at that time, was the series system. In this system current generated in the dynamo armature flowed through the field coils, out to one lamp after another over a wire, and then back to the dynamo. There were no means by which one lamp could be turned on and off without doing the same with all the others on the circuit. Edison realized that while this was satisfactory for street lighting where arcs were generally used, it never would be commercial for household lighting. He therefore decided that a practical incandescent electric lighting system must be patterned after gas lighting with which it would compete. He therefore made an intensive study of gas distribution and reasoned that a constant pressure electrical system could be made similar to that of gas. The first problem was therefore to design a dynamo that would give a constant pressure instead of constant current. He therefore reasoned that the internal resistance of the armature must be very low or the voltage would fall as current was taken from the dynamo. Scientists had shown that the most economical use of electricity from a primary battery was where the external resistance of the load was the same as the internal resistance of the battery, or in other words, 50 per cent was the maximum possible efficiency. [Illustration: DIAGRAM OF CONSTANT CURRENT SERIES SYSTEM. This, in 1878, was the only method of distributing electric current.] [Illustration: DIAGRAM OF EDISON’S MULTIPLE SYSTEM, 1879. Edison invented the multiple system of distributing electric current, now universally used.]